(Meth)acrylic acid is used as an abbreviation and represents acrylic acid or methacrylic acid. (Meth)acrylic acid, either as such or in the form of its esters, is particularly important for the preparation of polymers for various applications, for example for use as adhesives.
(Meth)acrylic acid is obtainable, inter alia, by catalytic gas-phase oxidation of alkanes, alkanols, alkenes or alkenals of 3 or 4 carbon atoms. (Meth)acrylic acid is particularly advantageously obtainable, for example, by catalytic gas-phase oxidation of propene, acrolein, tert-butanol, isobutene, isobutane, isobutyraldehyde or methacrolein.
However, other possible starting compounds are those from which the actual C.sub.3 /C.sub.4 starting compound is first formed as an intermediate during the gas-phase oxidation. An example is the methyl ether of tert-butanol.
These starting gases, as a rule diluted with inert gases, such as nitrogen, CO.sub.2, saturated hydrocarbons and/or steam, are passed as a mixture with oxygen, at elevated temperatures (usually from 200.degree. to 400.degree. C.) and, if required, superatmospheric pressure, over transition metal mixed oxide catalysts (containing, for example, Mo, V, W and/or Fe) and are converted by oxidation into (meth)acrylic acid (cf. for example DE-A 4 405 059, EP-A 253 409, EP-A 92 097, DE-A 44 31 957 and DE-A 44 31 949).
Owing to the many parallel and secondary reactions taking place in the course of the catalytic gas phase oxidation and due to the inert dilution gases to be used concomitantly, the product of the catalytic gas-phase oxidation is, however, not pure (meth)acrylic acid but a reaction gas mixture which contains essentially (meth)acrylic acid, the inert dilution gases and byproducts, from which the (meth)acrylic acid must be separated. In addition to byproducts which are comparatively simple to remove from (meth)acrylic acid and are less troublesome in subsequent applications of the (meth)acrylic acid, for example acetic acid, the reaction gas mixture also contains in particular lower aldehydes, such as formaldehyde, acetaldehyde, acrolein, methacrolein, propionaldehyde, n-butyraldehyde, benzaldehyde, furfural and crotonaldehyde, which are related to (meth)acrylic acid and therefore more difficult to separate from (meth)acrylic acid, and in addition may contain maleic anhydride (the total amount of these secondary components, which frequently present considerable problems during subsequent applications, is as a rule .ltoreq.2% by weight, based on the amount of (meth)acrylic acid contained in the reaction gas mixture).
The (meth)acrylic acid is usually isolated from the reaction mixture by extraction and rectification processes, ie. as a rule the (meth)acrylic acid formed is initially taken up from the reaction gas mixture of the gas-phase oxidation into a suitable absorbent. By separation of the absorbate by rectification, a crude (meth)acrylic acid is then usually obtained, from which a pure (meth)acrylic acid is frequently produced by passing through further separation stages involving rectification. The crude (meth)acrylic acid has, as a rule, a purity of &gt;98% by weight, the impurities being in particular lower aldehydes and possibly maleic anhydride, whereas the separation of the (meth)acrylic acid from the absorption liquid takes place essentially quantitatively. In contrast to crude (meth)acrylic acid, the purity of the pure (meth)acrylic acid is usually &gt;99% by weight.
DE-A 44 36 243 relates, for example, to a process for the isolation of (meth)acrylic acid from the reaction gas mixture of the catalytic gas-phase oxidation by countercurrent absorption with a high-boiling inert hydrophobic organic liquid, in which the reaction gas mixture is passed countercurrent to the descending high-boiling inert hydrophobic organic liquid in an absorption column, a rectification process is superposed on the absorption process taking place in the absorption column in a natural manner by withdrawing from the absorption column an amount of energy over and above its natural energy release taking place as a result of contact with the ambient temperature, and crude (meth)acrylic acid is isolated via the top by rectification from the liquid discharge of the absorption column (absorbate), which discharge contains (meth)acrylic acid and the absorbent as main components and lower aldehydes and possibly maleic anhydride as secondary components. DE-A 44 36 243 defines high-boiling inert hydrophobic organic liquids (absorbents) generally as those liquids whose boiling point at atmospheric pressure (1 atm) is above the boiling point of (meth)acrylic acid and which comprise at least 70% by weight of molecules which do not contain any externally acting polar group and are thus, for example, incapable of forming hydrogen bridges.
German Patent 2,136,396 and DE-A 43 08 087 likewise disclose the isolation of acrylic acid from the reaction gas mixture of the catalytic gas-phase oxidation of propylene and/or acrolein by countercurrent absorption with a high-boiling inert hydrophobic organic liquid. The process is essentially carried out by a method in which the reaction gas mixture is passed countercurrent to the descending absorption liquid in a conventional absorption column, the readily volatile secondary components which are easy to separate off are substantially removed, by stripping with inert gas in a desorption column, from the liquid discharge of the absorption column, which discharge is composed essentially of acrylic acid, the absorbent and secondary components, and the liquid discharge of the desorption column, which discharge contains acrylic acid and the absorbent as main components and lower aldehydes and possibly maleic anhydride as secondary components, is subsequently rectified in a rectification column in order to isolate crude acrylic acid via the top.
EP-A 297 445 relates, for example, to a process for the isolation of methacrylic acid from the reaction gas mixture of the catalytic gas-phase oxidation by absorption of the methacrylic acid in an absorption column operated with water. For isolation of crude methacrylic acid as a bottom product, the liquid discharge of the absorption column is rectified.
EP-A 117 146 relates to a process for the isolation of acrylic acid from the reaction gas mixture of the catalytic gas-phase oxidation by absorption of the acrylic acid in an absorption column operated with water. The acrylic acid is separated off from the liquid discharge by extraction with ethyl acetate and crude acrylic acid is obtained from the extract as a bottom product by rectification.
EP-B 102 642, British Patent 1,346,737 and DE-B 2 207 184 relate to the isolation of pure (meth)acrylic acid from crude (meth)acrylic acid by rectification. In order to increase the separation efficiency with respect to lower aldehydic impurities, it is advisable to add chemical compounds, such as primary amines, which bind the aldehydic impurities.
The abovementioned isolations of (meth)acrylic acid from a mixture containing (meth)acrylic acid as the main component and lower aldehydes as secondary components by rectification in a rectification column consisting of a stripping section and a rectification section are merely a small selection from the variety of such separation problems mentioned in the prior art.
A disadvantage of all these prior art separation processes by rectification, regardless of whether the (meth)acrylic acid is isolated via the top or via the bottom, is that the rectification apparatuses (in particular the evaporator surface) relatively rapidly become covered with a coating during the isolation by rectification, even when polymerization inhibitors, such as air, hydroquinone, hydroquinone monomethyl ether, para-nitrosophenol, para-methoxyphenol and/or phenothiazine, are present, and the rectification, which is usually carried out continuously, therefore has to be stopped from time to time in order to remove the coatings formed. The different colors of the coatings formed (black and white) show that at least two processes are involved in the formation of the coatings.